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1
Special equipment: a 3-quart baking dish
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Bring a large pot of salted water to a boil.
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Add the green beans, and cook until bright green and crisp-tender, about 10 minutes.
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4
Drain and shake off any excess moisture; set aside.
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5
Melt the butter in a medium pot over medium heat.
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Add the onions, and cook, stirring, until soft, about 8 minutes.
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Add the mushrooms and a pinch of salt, and continue to cook, stirring, until they are soft and most of their liquid has evaporated, about 10 minutes.
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8
Sprinkle the flour over the vegetables, and stir until all the flour has been absorbed, about 1 minute.
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9
Add the chicken broth, half-and-half, 3/4 teaspoon salt and several grinds of pepper.
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10
Bring to a boil, then reduce to a simmer and cook, stirring, until the sauce thickens and coats the back of a spoon, about 5 minutes.
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11
Remove the pot from the heat, add the green beans and stir to coat in the sauce.
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12
Transfer the mixture to a 3-quart baking dish.
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13
Let the casserole cool to room temperature.
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14
Wrap well with plastic wrap and foil, label and date, then freeze for up to 2 weeks.
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15
To serve, preheat the oven to 350 degrees F. Discard the plastic wrap and foil from the frozen casserole.
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16
Cover with new foil, and bake until warmed through, about 45 minutes.
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17
Uncover, and top with the fried onions.
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18
Continue to bake until the casserole is hot and bubbly and the onions have crisped slightly, about 20 minutes more.
